An ever increasing number of devices, such as the computer modem, the answering machine, and the facsimile machine, are linked to a network which was conceived and configured to accommodate a single type of device having a single use, namely, the voice telephone. Thus, the "telephone network" has evolved into a "telecommunications network."
The telecommunications network permits transmission of a diverse set of signals capable of supporting the various requirements of telecommunications devices. Exemplary signals include: a direct current voltage; an alternating current ring signal voltage; a dial tone signal; and communication signals having device identification tones. Of course, the signals can also include line noise, spikes, and other transient signals which can interfere with communication.
In certain situations, it can be desirable to have separate links to the network (phone lines) for each telecommunication device. However, due to cost and other considerations, more than one device commonly shares a line with another device, wherein each device is at a different location or extension. Therefore, when a ring signal, nominally a 20 Hz, 90 Vrms AC signal having a ring cycle of approximately 50 milliseconds, is received from the network, all extensions ring. If, for example, a telephone and a facsimile machine share a line and a ring signal is received, the telephone rings and is answered to determine the nature of the incoming call. If the distinctive facsimile identification tone is heard, the person answering the telephone activates the facsimile machine and hangs up the telephone.
Line switching devices exist to direct incoming communication signals to a telecommunications device sharing a line with a telephone so that a person does not need to answer the telephone and manually select another device. A typical line switching device operates by detecting a ring signal, identifying a distinctive identification tone, and activating an appropriate device. When a line switching device is in series with a telephone, the switching device can detect a ring signal before the telephone does. However, this approach is not practical when there are multiple extension telephones as a switching device must be associated with each telephone or be installed at the telephone line entry to the building.
Instead of being wired in series, telephone extensions sharing a single line are generally connected in parallel so that incoming signals are provided to each of the extensions at substantially the same time. However, known line switching devices operate so slowly that when a telephone shares a line with a telephone answering device (TAD) or a facsimile machine (FAX) (each at a different extension), the telephone rings, even though the TAD or the FAX is eventually activated. This can be particularly annoying if the TAD or FAX receives calls frequently or if calls are received late at night in the home.
An example of the type of line switching device that can be used in a shared line situation, but which still allows ringing at all extensions, is disclosed in U.S. Pat. No. 4,939,775 to Houck et al. In order to identify a distinctive ring signal, the Houck device must monitor an incoming ring signal for at least one complete ring cycle. However, the completion of a ring cycle will cause a telephone on an extension to ring.